RE: [Xen-devel] [PATCH] xen/privcmd: fix static checker warning

[Paul Durrant]

> -----Original Message-----
> From: Andrew Cooper
> Sent: 07 June 2018 11:28
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> Cc: Juergen Gross <jgross@xxxxxxxx>; Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx>; Dan Carpenter
> <dan.carpenter@xxxxxxxxxx>
> Subject: Re: [Xen-devel] [PATCH] xen/privcmd: fix static checker warning
> 
> On 07/06/18 11:21, Paul Durrant wrote:
> > Commit 3ad0876554ca ("xen/privcmd: add
> IOCTL_PRIVCMD_MMAP_RESOURCE")
> > introduced a static checker warning:
> >
> >   drivers/xen/privcmd.c:827 privcmd_ioctl_mmap_resource()
> >   warn: passing casted pointer 'pfns' to 'xen_remap_domain_mfn_array()'
> >         64 vs 32.
> >
> > caused by this cast:
> >
> >    827  num = xen_remap_domain_mfn_array(vma,
> >    828                                   kdata.addr & PAGE_MASK,
> >    829                                   pfns, kdata.num, (int *)pfns,
> >                                                           ^^^^^^^^^^^
> >
> > The reason for the cast is that xen_remap_domain_mfn_array() requires
> an
> > array of ints to store error codes. It is actually safe to re-use the
> > pfns array for this purpose but it does look odd (as well as leading to
> > the warning). It would also be easy for a future implementation change
> > to make this re-use unsafe so this patch modifies privcmd to use a
> > separately allocated array for error codes.
> >
> > Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> 
> It may be safe to reuse pfns[] as the storage space for the errs array,
> but code is incorrect when sizeof(pfn) != sizeof(int). In such a case,
> you skip over every other err, and second half of pfns[] is junk from
> the point of view of the errs loop.
> 

Yep, that is indeed what happens without this patch.

  Paul

> ~Andrew
> 
> > ---
> > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > Cc: Juergen Gross <jgross@xxxxxxxx>
> > Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> > ---
> >  drivers/xen/privcmd.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> > index 8ae0349..8507c13 100644
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -822,11 +822,18 @@ static long privcmd_ioctl_mmap_resource(struct
> file *file, void __user *udata)
> >  		unsigned int domid =
> >  			(xdata.flags & XENMEM_rsrc_acq_caller_owned) ?
> >  			DOMID_SELF : kdata.dom;
> > +		int *errs;
> >  		int num;
> >
> > +		errs = kcalloc(kdata.num, sizeof(*errs), GFP_KERNEL);
> > +		if (!errs) {
> > +			rc = -ENOMEM;
> > +			goto out;
> > +		}
> > +
> >  		num = xen_remap_domain_mfn_array(vma,
> >  						 kdata.addr & PAGE_MASK,
> > -						 pfns, kdata.num, (int *)pfns,
> > +						 pfns, kdata.num, errs,
> >  						 vma->vm_page_prot,
> >  						 domid,
> >  						 vma->vm_private_data);
> > @@ -836,12 +843,14 @@ static long privcmd_ioctl_mmap_resource(struct
> file *file, void __user *udata)
> >  			unsigned int i;
> >
> >  			for (i = 0; i < num; i++) {
> > -				rc = pfns[i];
> > +				rc = errs[i];
> >  				if (rc < 0)
> >  					break;
> >  			}
> >  		} else
> >  			rc = 0;
> > +
> > +		kfree(errs);
> >  	}
> >
> >  out: